It is the fifth set of missile tests in less than three weeks, and is being seen as a protest at the slow pace of nuclear talks with the US, and the ongoing US-South Korea joint military exercises in the region.
The launches were revealed hours after Donald Trump said he received a "beautiful" letter from North Korean leader Kim Jong Un, and predicted that they will engage in further talks to try and resolve the nuclear stand-off.
Mr Trump reiterated that he was not concerned by the weapons being launched, despite the threat they pose to the US allies in the region.
South Korea's joint chiefs of staff said that the alleged ballistic missiles were fired from the North's eastern coast - travelling about 250 miles and reaching a height of 30 miles.
They landed in waters between the Korean Peninsula and Japan.
Officials in Seoul said that the tests were likely aimed at confirming how reliable the North's newly developed weapons were, and demonstrating their resentment over the US-South Korea military drills.
Pyongyang has launched a number of short-range missiles over the last few weeks, saying that the allied military drills compel it to "develop, test and deploy the powerful physical means essential for national defence".
Mr Trump has been downplaying the secretive state's launches, with experts saying this tactic has allowed the country to intensify its testing activity, whilst it seeks to build leverage ahead of negotiations, which could start again sometime after the end of drills later in the month.
Leif-Eric Easley, an expert at Seoul's Ewha Womans University, said that the North is also seeking to exploit Mr Trump's preoccupation with getting South Korea to pay more for US troop deployment in the country, as well as Seoul's deteriorating relationship with Japan.
South Korea has threatened to end a military intelligence sharing agreement with Tokyo, in what is thought to be an attempt to pressure the US to intervene in the dispute.
Mr Easley said: "Kim appeals to Trump directly about the exercises, trying to drive a wedge between Washington and Seoul.
"Meanwhile, North Korean propaganda supports rising anti-Japan sentiment in South Korea, calculating that a diplomatically isolated Seoul will be more subject to Pyongyang's coercion."
The military drills have been scaled down since the first summit between Mr Trump and Mr Kim in June 2018 in Singapore created space for diplomacy - but the North Korea insists that even the reduced drills violate an agreement between the pair.
However, the recent tests in the North have dampened optimism that followed the third and most recent meeting between the US and North Korean leaders in June at the Korean border.
They have since agreed to resume working-level nuclear talks, but no known meetings have yet taken place since.